The present invention relates to an improved platen for use in an impact printer. More particularly, the present invention relates to such a platen capable of reducing noise by absorbing vibrations that occur upon a print head of the printer striking a printing surface on the platen.
A conventional platen for an impact printer comprises a steel shaft having on an outer surface thereof a hard covering or layer of ebonite or other material. However, such conventional platen is very heavy, and this requires that the printer include a large driving motor for rotating the platen. Further, when a print head of the printer strikes the platen, substantial and large noise is generated. In view of such problems of the conventional platen, various attempts have been made to devise improved platens that are lighter in weight and that reduce noise upon striking by the printer print head.
For example, one attempted improvement is to replace the steel shaft with a hollow steel pipe. This arrangement reduces the weight of the platen, but the noise generated during printing is increased due to the resonance phenomenon of the hollow pipe.
One proposal to reduce such resonance phenomenon is disclosed in Japanese Patent Application Laid-Open No. 3975/1988. This arrangement fills the hollow pipe with liquid having a low viscosity. This solution reduces noise from the resonance phenomenon, but other noise newly is generated. Particularly, such platen cannot absorb efficiently vibrations imparted to the platen, and the platen acts as a transmitter of such vibrations. Another solution, disclosed in Japanese Patent Application Laid-Open No. 204973/1987 fills approximately 50% of the interior of the hollow pipe with granular material such as sand. This solution absorbs impact occurring on printing to reduce noise. However, this solution does not remove noise due to the resonance phenomenon because a portion of the interior of the hollow pipe is not filled, i.e. there is a space therein enabling movement of the granular material.
As a solution for such problems, the present inventor has provided a platen, disclosed in Japanese Patent Application Laid-Open No. 173725/1992. Such platen provides that the interior of the hollow pipe is filled with liquid and sheet or granular fragments within the liquid that rub against each other. This platen absorbs impacts during printing by means of inertia resistance of the mass of the many fragments and the liquid, frictional resistance occurring upon the fragments rubbing against each other through the liquid, and velocity resistance of the liquid. Therefore, vibrations that occur upon the print head striking the printing surface on the platen can be prevented, thereby reducing noise of the platen as well as noise of the printer frame and other parts of the printer due to transmission of such vibration. Such platen employs as the filler liquid for preventing vibrations an emulsion paint, such as Gelnac (trade name) made by Nippon Automatic Co., Ltd. However, the use of such material results in certain inherent disadvantages. Thus, such filler requires a drying process involving a long period of time, and this adversely effects productivity. Further, such filler is hard to treat and is high priced. Due to such disadvantages, it is difficult to produce a low priced printer employing such platen.